1. Field of the Invention
The present invention relates to a magneto-optical recording medium, and more particularly, it relates to an improvement in a magneto-optical recording medium for reproducing a signal by enlarging a magnetic domain.
2. Description of the Related Art
A magneto-optical recording medium, which is noted as a reloadable and highly reliable recording medium having a large storage capacity, is put into practice as a computer memory or the like. Further, standardization of a magneto-optical recording medium having a storage capacity of 6.1 Gbytes at a diameter of 12 cm is recently progressed.
In addition, a magnetic domain enlarging/reproducing technique of applying an alternating magnetic field in signal reproduction from a magneto-optic recording medium for reproducing a signal by enlarging a magnetic domain transferred from a recording layer to a reproducing layer with the alternating magnetic field is developed, and a magneto-optical recording medium which can record and/or reproduce a signal of 14 Gbytes in/from a disk of 12 cm in diameter through this technique is also proposed.
In the signal reproduction employing such a magnetic domain enlarging/reproducing technique, the magnetic domain of the recording layer recording the signal is transferred to the reproducing layer so that the transferred magnetic domain is enlarged by the externally applied alternating magnetic field and reproduced. In this case, the magnetic domain is transferred from the recording layer to the reproducing layer by exchange coupling or magnetostatic coupling. In case of transferring the magnetic domain by magnetostatic coupling, a non-magnetic layer is provided between the recording layer and the reproducing layer so that the magnetic domain is transferred from the recording layer to the reproducing layer when the intensity of a leakage magnetic field reaching the reproducing layer from the magnetic domain exceeds coercive force reduced as the temperature of the reproducing layer is increased.
When transferring the magnetic domain from the recording layer to the reproducing layer by magnetostatic coupling, therefore, the thickness of the non-magnetic layer must be reduced to some extent in order to increase the intensity of the leakage magnetic field reaching the reproducing layer from the recording layer.
If the thickness of a non-magnetic layer 16 is reduced as shown in FIG. 1A, however, a leakage magnetic field reaching a reproducing layer 15 from a recording layer 17 through the non-magnetic layer 16 is strongly distributed as shown by arrows 150 around peripheral portions of its domain as shown in FIG. 1B, while leakage magnetic fields 152 opposite to the magnetization direction of the magnetic domain to be reproduced are formed outside the domain, and the leakage magnetic fields 152 are as strong as the parts 150 of the leakage magnetic field distributed around the peripheral portions of the domain. Consequently, the leakage magnetic fields 152 prevent enlargement of the magnetic domain, and hence the magnetic domain transferred to the reproducing layer 15 is disadvantageously insufficiently enlarged by an externally applied alternating magnetic field.
When the thickness of a non-magnetic layer 16 is increased to some extent for distributing a leakage magnetic field so that the intensity thereof is maximized at the center of its domain, on the other hand, a part 140 of the leakage magnetic field around the center of its domain hardly reaches a reproducing layer 15 although leakage magnetic fields 141, opposite to the magnetization direction of a magnetic domain to be reproduced, formed outside the domain are reduced in intensity, as shown in FIGS. 2A and 2B. If the non-magnetic layer 16 has a large thickness, therefore, the magnetic domain is hardly transferred.
Accordingly, an object of the present invention is to provide a magneto-optical recording medium capable of readily transferring a magnetic domain from a recording layer to a reproducing layer and readily enlarging the transferred magnetic domain.
According to the present invention, the magneto-optical recording medium includes a reproducing layer, an intermediate layer formed on the reproducing layer, and a recording layer formed on the intermediate layer. The recording layer includes a first layer having first saturation magnetization and first coercive force and a second layer having second saturation magnetization larger than the first saturation magnetization at a reproducing temperature and second coercive force smaller than the first coercive force at the room temperature. Preferably, the second layer is made of an alloy of a noble metal and a transition metal. Alternatively, the second layer is formed by alternately stacking a first thin film made of a noble metal and a second thin film made of a transition metal. Alternatively, the second layer contains a transition metal and a metal having a magnetic moment ferromagnetically exchange-coupling with the transition metal.
In the aforementioned magneto-optical recording medium, the saturation magnetization of the second layer is larger than that of the first layer at the reproducing temperature, to increase the intensity of a leakage magnetic field reaching the reproducing layer from the recording layer through the intermediate layer. Therefore, a magnetic domain of the recording layer is reliably transferred to the reproducing layer and the transferred magnetic domain can be reliably enlarged. On the other hand, the coercive force of the first layer is larger than that of the second layer at the room temperature, and hence the recording layer can reliably maintain a signal recorded therein.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.